The detrusor muscle is a layer of the urinary bladder wall made of smooth muscle fibers arranged in spiral, longitudinal, and circular bundles. When the bladder is stretched, this signals the parasympathetic nervous system to contract the detrusor muscle. This encourages the bladder to expel urine through the urethra.
For the urine to exit the bladder, both the autonomically controlled internal sphincter and the voluntarily controlled external sphincter must be opened. Problems with these muscles can lead to incontinence. If the amount of urine reaches 100% of the urinary bladder's absolute capacity, the voluntary sphincter becomes involuntary and the urine will be ejected instantly.
The human adult urinary bladder usually holds about 300-350 ml of urine (the working volume), but a full adult bladder may hold up to about 1000 ml (the absolute volume), varying among individuals. As urine accumulates, the ridges produced by folding of the wall of the bladder (rugae) flatten and the wall of the bladder thins as it stretches, allowing the bladder to store larger amounts of urine without a significant rise in internal pressure.
In most individuals, the desire to urinate usually starts when the volume of urine in the bladder reaches around 200 ml. At this stage it is easy for the subject, if desired, to resist the urge to urinate. As the bladder continues to fill, the desire to urinate becomes stronger and harder to ignore. Eventually, the bladder will fill to the point where the urge to urinate becomes overwhelming, and the subject will no longer be able to ignore it.
In some individuals, this desire to urinate starts when the bladder is less than 100% full in relation to its working volume. Such increased desire to urinate may interfere with normal activities, including the ability to sleep for sufficient uninterrupted periods of rest. In some cases, this increased desire to urinate may be associated with medical conditions such as benign prostate hyperplasia or prostate cancer in men, or pregnancy in women. However, increased desire to urinate also occurs in individuals, both male and female, who are not affected by another medical condition.
In some individuals, such as in children, involuntary urination (e.g., bed wetting) may occur as a result of lack of control to the bladder muscle. In other individuals, involuntary urination (e.g., urinary incontinence) may occur as a result from an underlying medical condition.
Accordingly, there exists a need for compositions and methods for the treatment of male and female subjects who suffer from an undesired frequency of urination.